1. Field of the Invention
The present invention relates generally to ultra-wide band impulse transmission systems and more particularly to applying time-hopping codes (THC) to ultra-wide band impulse transmission systems.
2. Related Art
For numerous reasons both technological and regulatory including, e.g., interference rejection and suppression, it is desirable that transmitters such as, e.g., Ultra Wideband (UWB) transmitters, be able to modify the spectral properties of the pulse trains they emit through the use of time-hopping codes (THC) to achieve a desirable spectral response.
As the availability of communication bandwidth in the increasingly crowded frequency spectrum is becoming a scarce and valuable commodity, UWB technology provides an excellent alternative for offering significant communication bandwidth, particularly, for various wireless communications applications. Because UWB communication systems are based on communicating extremely short-duration pulses (e.g., pico-seconds in duration), such systems are also known as impulse radio systems. Impulse radio systems were described in a series of patents, including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990), and U.S. Pat. No. 5,363,057 (issued Nov. 8, 1994) to Larry W. Fullerton, and U.S. Pat. No. 5,677,927 (issued Oct. 14, 1997), U.S. Pat. No. 5,687,169 (issued Nov. 11, 1997), and U.S. Pat. No. 5,832,035 (issued Nov. 3, 1998) to Larry W. Fullerton, et al. These patents are incorporated herein by reference in their entireties.
Multiple access impulse radio systems are radically different from conventional Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) systems. Unlike such systems, which use continuous sinusoidal waveforms for transmitting information, a conventional impulse radio transmitter emits a low power electromagnetic train of short pulses, which are shaped to approach a Gaussian monocycle. As a result, the impulse radio transmitter uses very little power to generate noise-like communication signals for use in multiple-access communications, radar and positioning applications, among other things. In the multi-access communication applications, the impulse radio systems depend, in part, on processing gain to achieve rejection of unwanted signals. Because of the extremely high achievable processing gains, the impulse radio systems are relatively immune to unwanted signals and interference, which limit the performance of systems that use continuous sinusoidal waveforms. The high processing gains of the impulse radio systems also provide much higher dynamic ranges than those commonly achieved by the processing gains of other known spread-spectrum systems.
Impulse radio communication systems transmit and receive the pulses at precisely controlled time intervals, in accordance with a time-hopping code. As such, the time-hopping code defines a communication channel that can be considered as a unidirectional data path for communicating information at high speed. In order to communicate the information over such channels, typical impulse radio transmitters use position modulation, which is a form of time modulation, to position the pulses in time, based on instantaneous samples of a modulating information signal. The modulating information signal may for example be a multi-state information signal, such as a binary signal. Under this arrangement, a modulator varies relative positions of a plurality of pulses on a pulse-by-pulse basis, in accordance with the modulating information signal and a specific time-hopping code that defines the communication channel.
In applications where the modulating information signal is a binary information signal, each binary state may modulate the time position of more than one pulse to generate a modulated, coded timing signal that comprises a train of identically shaped pulses that represent a single data bit. The impulse transmitter applies the generated pulses to a specified transmission medium, via a coupler, such as an antenna, which electromagnetically radiates the pulses for reception by an impulse radio receiver. The impulse radio receiver typically includes a single direct conversion stage. Using a correlator, the conversion stage coherently converts the received pulses to a baseband signal, based on a priori knowledge of the time-hopping code. Because of the correlation properties of the selected time-hopping codes, the correlator integrates the desired received pulses coherently, while the undesired noise signals are integrated non-coherently such that by comparing the coherent and non-coherent integration results, the impulse receiver can recover the communicated information.
Conventional spread-spectrum code division multiple access (SS-CDMA) techniques accommodate multiple users by permitting them to use the same frequency bandwidth at the same time. Direct sequence CDMA systems employ pseudo-noise (PN) codewords generated at a transmitter to “spread” the bandwidth occupied by transmitted data beyond the minimum required by the data. The conventional SS-CDMA systems employ a family of orthogonal or quasi-orthogonal spreading codes, with a pilot spreading code sequence synchronized to the family of codes. Each user is assigned one of the spreading codes as a spreading function. One such spread-spectrum system is described in U.S. Pat. No. 4,901,307 entitled SPREAD-SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS by Gilhousen et al.
Unlike direct sequence spread-spectrum systems, the time-hopping code for impulse radio communications is not necessary for energy spreading, because the monocycle pulses themselves have an inherently wide bandwidth. Instead, the impulse radio systems use the time-hoping codes for channelization, energy smoothing in the frequency domain, and interference suppression. The time-hoping code defines a relative position of each pulse within a group of pulses, or pulse train, such that the combination of pulse positions defines the communications channel. In order to convey information on such communications channel, each state of a multi-state information signal may vary a relative pulse position by a predefined time shift such that a modulated, coded timing signal is generated including a train of pulses, each with timing corresponding to the combination of the time position coding and the multi-state modulation. Alternatively, or additionally, other modulation techniques may be employed.
In one conventional binary approach, pulses are time-modulated forward or backward about a nominal position. More specifically, each pulse is time modulated by adjusting its position within a time frame to one of two or more possible times. For example, in order to send a “0” binary bit during the time frame, the pulse may be offset from a nominal position of the time frame by about −50 pico-seconds. For a “1” binary state, the pulse may be offset from the nominal position by about +50 pico-seconds. Conventional coders that generate the time-hoping code do so in response to a periodic timing signal that corresponds to the data-rate of the multi-state information signal. The data rate of the impulse radio transmission may for example be a fraction of a periodic timing signal that is used as a time base or time reference.
In practice, decoding errors are minimized using distinctive time-hopping codes with suitable autocorrelation and cross-correlation properties. The cross-correlation between any two time-hopping codes should be low for minimal interference between multiple users in a communications system or between multiple target reflections in radar and positioning applications. At the same time, the autocorrelation property of a time-hoping code should be steeply peaked, with small side-lobes. Maximally peaked time-hopping code autocorrelation yields optimal acquisition and synchronization properties for communications, radar and positioning applications.
Various coding schemes with known correlation characteristics are available. For example, algebraic codes, Quadratic Congruential (QC) codes, Hyperbolic Congruential (HC) codes and optical codes have been suggested in the past for coding in impulse radio systems. Generally, based on known assumptions, the coding schemes guarantee a maximum number of pulse coincidences, i.e., hits, for any defined time frame or time frame shift during which the codes are repeated. For example, HC codes are guaranteed a maximum of two hits for any sub-frame or frame shift.
McCorkle in U.S. Pat. No. 5,847,677 discloses a random number generator for generating a pseudo-random code for use with jittered pulse repetition interval radar systems. The code is generated by a random number generator that possesses certain attributes desirable for a jittered radar. As disclosed, the attributes related to a flat frequency spectrum, a nearly perfect spike for an autocorrelation function, a controllable absolute minimum and maximum interval, long sequences that do not repeat, and a reasonable average pulse rate.
One known coding technique for an impulse radio is disclosed by Barrett in U.S. Pat. No. 5,610,907, entitled “Ultrafast Time Hopping CDMA-RF Communications: Code-As-Carrier, Multichannel Operation, High data Rate Operation and Data Rate on Demand.” According to the disclosed techniques, two levels of coding are used: major orthogonal codes are applied to provide multiple channels, and forward error correction (FEC) codes are applied to information data before transmission. The disclosed system relies on dividing time into repetitive super-frames, frames and sub-frames. As disclosed, a super-frame corresponds to a time interval of about 1 millisecond, representing one repetition of a code pattern, where as a frame is defined as a time interval of about 1 microsecond divided according to a code length. A sub-frame corresponds to a short time interval of about 1 nano second during which a pulse is time positioned.
It is desirable that systems, methods, computer program products and apparatuses be provided that could prevent a radio transceiver from interfering with or being interfered with by narrow band systems.
Regarding transmitters, it is desirable to prevent interfering with other transmitted signals in a particular frequency band, such as, e.g., frequencies in which, e.g., air traffic control systems, global positioning system (GPS), or medical equipment communicate.
Regarding receivers, if a device is emitting a signal, broadcasting on a particular frequency band near a receiver, such as, e.g., a cell phone at 2.4 MHz, it would be desirable to avoid having the receiver be interfered by the signal. For example, it might be desirable to notch out a frequency between the 1 MHz and 3 MHz frequency band to avoid interference. Similarly, in the case of Personal Communication System (PCS) phone systems which communicate at the 1.8 to 1.9 MHz frequencies, it could be desirable to provide a notch in the spectrum at the 1.8 to 1.9 MHz frequency band.
It is also desirable, in the case of radar systems, to provide transceivers that avoid frequency bands of a jamming radar system. For example, where a jammer attempts to interfere with a radio, it is desirable to selectively notch out the interfering jamming radar signal.
U.S. Pat. No. 6,002,708 ('708) to “Spread Spectrum Localizers,” to Fleming et al., filed May 23, 1997, and U.S. Pat. No. 5,748,891 ('891) to “Spread Spectrum Localizers,” to Fleming et al., filed Jul. 22, 1994, the contents of which are incorporated herein by reference in their entirety, disclose a localizer that uses cancellation nulling using pairs of opposite polarity pulses separated by a fixed period of time.
The '708 and '891 patents disclose localizers that use cancellation nulling of multiple pairs of adjacent opposite polarity pulses where the time difference between the opposite polarity pulses in a pair of pulses must be a fixed time period to in the time domain, for each of the pairs. The localizers of the '708 and '891 patents, require that the pulses in a pair of pulses must have opposite polarity. Also, the distances between the two opposite polarity pulses of a pulse pair must remain constant among all pairs of pulses. Thus, unfortunately, the '708 and '891 patents do not provide for a method of canceling pulses that allows for all pulses to be of the same polarity. Also, the '708 and '891 patents do not allow for non-constant time differences between pulses in a pulse pair.
Therefore, there exists a need for a system and method of selectively shaping or notching out particular frequency bands in spectra that overcomes the shortcomings of conventional solutions.